I record all of my home demos on GB - on an iPhone no less!
You can get useable sounds from the built in amps. Clean tones especially. But distorted tones too, if you know a few tricks. Mainly I blend a distorted tone (something like Woodstock fuzz) with a cleaner tone, perhaps Classic Crunch or something.
The really heavy metal amp on the iOS version is prone to clipping, so you need to tone down the input levels, adjust the gain settings, and use the noise gate. But it is possible. I did this on GB iOS and I think it sounds pretty good:
https://www.youtube.com/watch?v=SG8s1nT-gF4
If only I could get to the end of it!
But anyway, the amps are ok for messing around with and doing quick demos.
My preference though, is to simply use the Red Box out on my Hughes and Kettner Tubemeister head, into an audio track in Garage Band. That's as close to a proper amp sound as you can get.